In practice, overhead contact wires are never perfectly rectilinear.
Firstly, public transport electric vehicles are often installed in urban areas where, for most of the time, they have to follow traffic paths which are intended for other types of vehicle.
Secondly, an effort is made during installation to ensure that the overhead wire is not rectilinear so that the device which provides electrical connection between the overhead conductor and the moving receiver, generally a pantograph, does not always rub at the same point on the catenary, thereby avoiding excessive wear of the pantograph contact plate.
For these reasons it is necessary to provide systems capable of counteracting or balancing the radial tension applied to the contact wire at curved portions thereof.
Accompanying FIG. 1 thus shows a conventional system for balancing the radial tension imposed on a contact wire. Such a system is shown, for example, in the publication "S.N.C.F.--LIGNES AERIENNES DE TRACTION ELECTRIQUES EN COURANT MONOPHASE 25 KV--50 Hz--PRINCIPE ET MATERIEL--1979", page 127, FIG. 122 (OVERHEAD ELECTRIC TRACTION LINES FOR SINGLE PHASE 25 KV--50 Hz, BY FRENCH STATE RAILWAYS).
FIG. 1 shows a generally vertical post 1 to which a generally horizontal bracket 2 is fixed. The bracket is further supported by a stay 4 connecting the top of the post 1 and the free end of the bracket 2.
The contact wire is referenced 10 in FIG. 1. The contact wire 10 is supported by means of a carrier cable 5 having its mid point 7 connected to the end of bracket 2 and having its ends connected to the contact wire 10 on either side of the bracket 2 by means of respective clamps 8.
As has been mentioned above, the system must also be adapted to balance the radial tension F.sub.R imposed on the contact wire. More precisely, the system must be suitable for balancing the tension F.sub.R which results from the weight F.sub.p of the length of contact wire 10 corresponding to the two half-spans on either side of the post 1 together with a pure radial tension F.sub.r imposed on the contact wire 10 in the horizontal plane thereof and relative to the axes of the path or of the rolling stock.
To do this, as has been shown in FIG. 1, the conventional solution consists in providing a return arm 9 which is hinged to the bracket 2 by a hing 11, whereby the return arm is free to pivot in a substantially vertical plane. Further, the free end of the return arm is connected at 12 to the contact wire 10 by means of a clamp. More precisely, the pivot point 11 of the return arm 9 on the bracket 2 and the length of the return arm 9 are determined in such a manner that the return arm 9 is situated substantially along the line of action of the resultant radial tension F.sub.R mentioned above.
Furthermore, as is explained in the above-mentioned publication, the point 7 where the carrier cable 5 is attached to the bracket 2 is situated vertically above the clamp 12 connecting the return arm 9 to the contact wire 10.
In practice, it is observed that such return arms for balancing the radial tension imposed on the contact wire 10 need to be installed on practically every suspension point.
Naturally, it will readily be understood that such installation is relatively expensive, particularly since in many applications double isolation is required between the contact wire and ground, which often means that the return arm must be made of electrically insulating material. Further, the weight of the return arm, and the radial tension acting on a single point of the contact wire, a "hard point" of reduced flexibility is created in the contact wire beneath the bracket 2. Furthermore, such return arms are not very satisfactory from the aesthetic point of view.
One proposal for solving this problem has been to do without the return arm 9 and to support and return the contact wire 10 by means of a support cable 5 which is connected by its mid point to the bracket 2 and which is connected to the contact wire 10 on either side of the bracket 2 by means of clamps which are clamped to the contact wire 10 and which include respective suspension arms that pivot about a vertical axis with the ends of the support cable 5 being connected to the free ends of the said arms.
In such a case, the point at which the carrier cable is connected to the bracket is no longer vertically above the contact wire.
More precisely, depending on the angle through which the contact wire is deviated, the suspension arm is pivoted about the vertical axis through an angle such that the traction force due to the carrier cable 5 passes through the clamping point of said clamps.
Such an arrangement is not entirely satisfactory, firstly since the suspension arms are subjected to high stress and the assembly is thus not very reliable, and secondly because it is necessary to adjust the length of the carrier wire 5 on site as a function of the degree of curvature, and thus to crimp the ends of the carrier cable outside a workshop which is long, expensive and, again, not very reliable.
A similar system is described in Swiss Pat. No. 454 209 with reference to its FIG. 2 which is mentioned as being representative of the state of the art. This Swiss patent further describes a solution in which the clamps are fitted with shackles which are in turn connected to a support by means of drop arms.
As stated in the text of said Swiss patent, the shackle is inclined relative to the vertical in such a manner that the axis of the drop arm intersects the axis of the contact wire.
This requires the jaws of the clamps fitted to the contact wire to be loosened on site.
As a result, it is particularly difficult to adjust such an assembly since the clamps are only attached to the contact wire after adjustments have been made.
In practice, it has been observed that the solutions described above are not widely used because of the many drawbacks they entail, and that the vast majority of current installations use suspension systems of the type shown in FIG. 1.
As is also described in U.S. Pat. No. 1,402,000 and in French Pat. No. 997 755, proposals have also been made for suspension systems for overhead lines which include in combination jaws suitable for clamping to the contact wire and a drop arm associated therewith.
More precisely, the devices described in these documents are intended to be used for supporting a contact wire which is suspended from a catenary, i.e. for an electric power supply line which includes a continuous carrier cable.
In consequence, the drop arms connecting the contact wire to the carrying catenary cable are mounted perpendicularly to the contact wire and the carrier cable and are adjustable relative to the clamps in such a manner that the plane of symmetry of the clamps remains vertical regardless of the angle at which the drop arms may be inclined in vertical planes running perpendicularly to the planes of symmetry through the clamps.
In addition to requiring the clamps to be loosened for the angles to be adjusted as a function of the line geometry, such devices are inapplicable to suspension systems that do not include a continuous catenary carrier cable.
Preferred embodiments of the present invention provide a suspension system for an overhead contact wire which is much simpler and cheaper than prior art systems and which is also less ugly, stronger and more reliable. In particular, the invention does not require a continuous carrier cable, but only requires lengths of carrier cable in association with each support point.